Non-optical methods for bioassay are of interest to the interdisciplinary fields of biology, applied physics, and microelectronics. Magnetic micro/nano beads have been studied for use in cell/bio-molecule sensing as one non-optical method. While the magnetic behavior of magnetic micro/nano beads can be detected without using costly imaging systems, sensing magnetic micro/nano particles remains a challenging task. Various detection methods have been proposed to address this sensing challenge. Traditionally, superconducting quantum interference devices (SQUIDs), giant magnetoresistance (GMR) arrays and atomic force microscopy have been used for their high sensitivity. However, sensing methods such as those based on SQUIDs or GMR arrays typically cannot be fabricated with standard integrated processes as CMOS. Also, both SQUID and GMR array generally require relatively costly postprocessing steps. Moreover, GMR techniques need externally generated biasing magnetic fields. Such biasing magnetic fields can be provided using either permanent or electro-magnets, which leads to extra power-consumption, large form-factor and high cost. Moreover, the biasing magnetic fields need to be calibrated to set a correct orientation, which increases the difficulty of use.
Another prior art sensor technology, the Hall sensor, is available in CMOS processes. Hall sensors, however, need a relatively high power external biasing field, thus precluding most low power portable battery powered applications. The biasing magnetic fields also have all of the impediments mentioned before for GMR sensors. Hall sensor based systems are also generally unsuitable for use in micro-fluidic systems. For example, for optimum sensitivity, Hall sensors should have dimensions (sensor size and passivation layer thickness) on the order of the dimensions of the magnetic beads to be sensed. Such Hall dimensions typically limit the hall sensor to a small sensing area and preclude sensor compatibility over a range of sizes of magnetic particles of interest. Moreover, to achieve a close proximity between the Hall sensor sensing part and the magnetic samples, expensive post processes, such as etching, are needed.
Therefore, what is needed is a more efficient and flexible system and method for making micro/nano magnetic bead based cell/bio-molecule measurements.